1. Field of the Invention
The present invention relates to an optical disk on which information is recorded and reproduced by focused light and a recording/reproducing device for recording or reproducing information for the optical disk.
2. Discussion of Background
In recent years, an image information filing device has been used in which image information such as documents to be produced in large quantities are photoelectrically-converted by means of two-dimensional scanning, and the photoelectrically-converted image information is recorded or retrieved and reproduced so that it can be output as hard copies.
In such a recording/reproducing device that has conventionally been used, an optical disk is used for recording information spirally and the information is recorded and reproduced by an optical head which moves straight to radius direction of the optical disk by a linear motor.
However, in the record reproducing device in which an optical disk is used, as described above, a beam may not be irradiated to a proper position due to mounting error when performing focusing of an objective lens at an optical head. Therefore, it is necessary to correct the position of focus by applying a bias voltage (offset voltage) when amplifying the detection signal for focusing.
However, the above correction is merely performed by means of setting of the optimum adjust position at mounting (at delivery), that is, however, when the bit error rate is produced by actual operation, the optimum adjust position is different and the bit error rate is increased. Also, the margin for variation of optical disks or deviation of optical axis due to change of external environment, or displacement of the photodetector is narrow. Further, for the above correction, no problem arises when the optium adjust position is set where the bit error rate is smallest and is the same for recording and for reproducing information. However, in reality, the optimum adjust position at recording and the optimum adjust position at reproducing are different due to astigmatism which a semiconductor laser oscillator has, or change of injection direction of laser light by output power, and the problem as described above has arisen.
Further, an optical disk in an optical disk recording/reproducing device is different in nature depending on the kind of materials which manufacturers use. Organic or inorganic materials may be used as the material of the recording layer of an optical disk. For an optical disk where inorganic materials are used, there are two types, pit forming type and phase transition type, or bubble forming type. For an optical disk where organic materials are used, there are two types, pit forming type and molecular structure changing type. These various recording layers are different in the reflection factor of light depending on the difference of the method of producing a recording layer or materials. Also, there are layers where the reflection factor of light is different, that is, layers where the reflection factor of light at the recording unit for the recorded signals is high and layers where the reflection factor is low. Or, the threshold value of the power of light source used for irradiation of a light is different depending on the difference of recording layers. Therefore, conventionally, it is impossible to perform recording or reproducing by the same optical disk recording/reproducing device by means of an optical disk where the nature of a recording layer is different from other layers.